Thin film chain memory



Nov. 24, 1970 M. MAY

THIN FILM CHAIN MEMORY 3 Sheets-Sheet 1 Filed Oct. 20, 1967 v Eta-.2.

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United States Patent 3,543,251 THIN FILM CHAIN MEMORY Michael May, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 20, 1967, Ser. No. 676,802 Int. Cl. Gllc /04, 11/06, 11/08 US. Cl. 340-174 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to an improved thin film magnetic memory and particularly to a rotationally switched chain magnetic memory that is easily constructed and that operates either destructively or nondestructively with a minimum of driving current.

One of the problems associated with magnetic memo ries is the assembly of the components while providing desired switching characteristics. For thin film structures, not only is a suitable sense signal required but it is desirable that rapid rotational switching be provided with a minimum of driving current. Also an easily constructed thin film array that provides. reliable nondestructive read out without destroying the stored information would be desirable in the memory field.

SUMMARY OF THE INVENTION Briefly this is a chain memory that is switched rotationally with a minimum amount of driving current and that is subject to simplified and rapid construction. A conductor is coated on both sides with a thin film that may be either isotropic or with an established anisotropy. The spacing between the films or the film thickness is selected so that a desired demagnetization force, that is the force resulting from a discontinuity of dipole elements, is present to allow rotational switching with a minimum of switching current. The chain memory elements may be formed to operate either with shape anisotropy or with an established crystal anisotropy in different arrangements in accordance with the invention. Also the system may be operated in a nondestruc tive or a destructive read out mode in accordance with the principles of the invention.

It is therefore an object of this invention to provide a thin film memory that is capable of being easily and rapidly constructed.

It is a further object of this invention to provide a magnetic chain memory in which rapid rotational switching is provided with a minimum amplitude of driving current.

It is a still further object of this invention to provide a thin film memory structure that is capable of a reliable nondestructive read out operation.

It is another object of this invention to provide an im- 0 proved method and configuration for constructmg a 7 thin film memory.

3,543,251 Patented Nov. 24, 1970 It is still another object of this invention to provide a chain memory structure having controlled demagnetization properties while still providing reliable rotational switching.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features of the invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the accompanying description taken in connection with the accompanying drawings, in which like reference characters refer to like parts and in which:

FIG. 1 is a schematic perspective drawing of one arrangement of the magnetic chain memory in accordance with the principles of the invention;

FIG. 2 is a schematic perspective drawing of another arrangement of the magnetic chain memory in accordance with the invention;

FIG. 3 is a schematic perspective drawing for further explaining the structure and operation of a chain element of FIGS. 1 and 2; and

FIG. 4 is a schematic diagram of current and voltage as a function of time for further explaining the operation of the chain memory systems of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, a memory array 10 is shown having a first storage level 11 utilizing chain magnetic film storage elements such as 12, '14 and .1 6 in a word line 18 and chain magnetic film storage elements 20, 22 and 24 in a word line 26. Other storage levels such as 28 and 30 are provided for additional Word storage. When the chain memory in accordance with the invention is operated in a word organized format, a source 32 may provide word pulses through a conductive substrate such as '54 both during writing and reading and a source 34 may provide bit or digit current pulses which are passed through digit or bit conductors such as 36, 38, and 40 during writing. It is to be noted that the digit conductors such as 36, 38 and 40, which are digit-sense conductors, are sense windings during reading in the illustrated organization. Insulating mounting boards such as 39, 41 and 43 are provided between adjacent layers of the memory array and are maintained with a desired spacing. The digit conductors such as 36, 38 and 40 may have a suitable electrical insulation thereon.

'Each of the chain elements such as 16 includes an oblong opening 42 which may have an elongated circular shape with parallel sides, the opening being surrounded by legs 44 and 46. It is to be noted that the openings may have other configurations in accordance with the principles of the invention such as being oval or circular FIG. 2). Between each pair of chain elements such as 14 and 16 is a neck portion such as 48. Each element such as 16 includes thin films 50 and 52 deposited on opposite sides of the conductor 54 which may be copper or any suitable conductive material etched to the proper chain shape or configuration.

Before further explaining the chain memory relative to FIG. 1, the memory arrangement of FIG. 2 will be explained, which arrangement is similar to that of FIG. 1 except a circular chain configuration is utilized for chain elements such as 60, 62 and 64 of a word chain or line 6 5. The chain element 60 may have a circular opening 66 through which the digit conductor 36 passes, have circular legs 68 and 70 and have a neck portion such as 72 connecting adjacent elements such as 60 and 62. Each word chain such as includes thin films 74 and 76 on opposite sides of a conductor 78 similar to the arrangement of FIG. 1.

Referring now principally to FIGS. 1 and 3 the oblong shaped chain element 16 will be explained in further detail. During writing or recording, a binary one may be stored with a magnetic polarity shown by arrows 80, 82, 84 and 86, the storage being in both thin films 50 and 52. A binary Zero is stored in the element 16 with an opposite polarity indicated by arrows 88, 90, 92 and 94. For either a one or a Zero, the flux path is continuous around each of the elongated circularly shaped thin films 50 and 52. During writing, a word current pulse indicated by an arrow 96 is applied from the source 32 through the selected word conductor 54 to rotate the oriented dipoles in the two films so as to be temporarily radially oriented. The field resulting from the word current in the film 50 is shown by arrows 100, 102, 104 and 106 and in the film 52 is shown by arrows 108, 110, 112 and 1 14.

For writing a selected state into the element, a digit current indicated by an arrow 118 or 120 is applied through the digit lead 36 simultaneous with the word current for writing a respective one or a zero. In response to the current of the arrow 118, the magnetic dipoles are rotated to the one polarity of the arrows 80 and 82 and in response to the current of the arrow 120, the magnetic dipoles are rotated in the direction of the arrows 88 and 90, both states being oriented along the easy axis. Thus, rotational switching is provided in both of the films 50 and 52.

Reading nondestructively results from a word current as indicated by the arrow 96 which rotates the dipoles in the direction of the arrows 100, 104, 108 and 1 14. In regions 91 and 93, the dipoles remain substantially undisturbed in response to the controlled read current, and these regions help insure that the entire element returns to the state recorded therein. During reading, a signal is sensed on the digit line 36 having a polarity representative of the stored state. It has been found that the pair of thin films in accordance with the invention induces a relatively large voltage signal in the lead 36.

The thin films may be formed isotropically and with shape anisotropy providing orientation of the dipoles around the elongated circle ring. In order to form the isotropic structure the chain is rotated in a fixed magnetic field (not shown) during formation of the thin film by conventional methods such as sputtering in a vacuum or by electrolytic deposition. This field which may be of a gauss intensity for a nickel thin film, for example, overcomes any ambient fields that may be present so that the magnetic elements or dipoles are not oriented, resulting in an isotropic thin film. The shape anisotropy or orientation of the elements 16 (whether oblong, round or oval) ha been found to provide sufiicient magnetic orientation to an easy axis, that the structure is capable of performing thin film or rotational switching. Also in accordance with the principles of the invention, the thin films may be deposited with current passing through conductors which may be similar to the digit current conductors 36, to form a crystal anisotropy or alignment of the magnetic elements in a direction around the loop. The shape anisotropy adds to the crystal anisotropy in this type of a thin film structure.

The chain memory in accordance with the principles of the invention develops controlled demagnetizing forces at the edges of the films forming the storage elements, so that a minimum of current is required for the word current both during writing and reading. The demagnetizing force at the fields of the arrows 108 and 109 results from the noncontinuity of the aligned magnetic dipoles between the films 50 and 52 where the fields pass through the ambient substance which may be air or a suitable material as indicated by arrows 105 and 107. This demagnetizing force which is the magnetic reluctance to the dipole elements going into alignment as the poles at the edges of the films create a demagnetizing field, is established to be a minimum in the system of the invention by selecting the spacing 117 between the films at a relatively small value. At the same time the space 117 is maintained sufficiently large so that thin film or rotational switching may be reliably performed. Because of the low demagnetizing force in the path of the arrows 108 and .109, the word current may have a relatively small value while still providing rapid rotational switching. Another parameter that may be controlled within the scope of the invention is the thickness of the films 50 and 52 to provide a desired demagnetization force by controlling the shape anisotropy.

In a chain memory, for example, the thin films 50 and 52 may be percent nickel and 20 percent iron and each 2.5 microns thick on a 5 mil thick copper sheet. With this spacing, the demagnetizing force is calculated to be between 6.4 and 7.4 oersteds. This structure is with isotropic deposition and an oblong configuration as shown in FIGS. 1 and 3 so that only the shape anisotropy provided orientation of the magnetic elements.

It is to be noted that for desirable rotational switching, the crystal and shape anisotropy field (H must be equal to or greater than the magnetic field (H which produces domain wall movement, or the dipoles may not return to an orientation in the easy direction at the end of a switching operation. This requirement is found to be a practical consideration because inverted film where H H is diflicult to make without exhibiting undesirably high angular dispersion of the easy axis. In the system of the invention, shape anisotropy will provide satisfactory rotational operation, although crystal anisotropy may also be established in accordance with the invention. The storage elements of the invention provide a substantially square hysteresis loop along the direction of the easy axis.

The operation and structure of the system of FIG. 2 is substantially similar to that of FIG. 1 except circular chain elements are provided. This circular structure may be desirable for some assembly operations and this or any suitable chain configuration is within the scope of the invention. The oblong structure of FIGS. 1 and 3 has the advantage that long sections of film are provided for rotational switching near to the field developed by the digit lead such as 36 which improves the operation because at the branches such as 91 and 93, the stored state is indefinite during writing.

Referring now to FIG. 4 as well as to FIGS. 1, 2 and 3 the operation of the chain magnetic memory system will be further explained. At a. time, t a read current pulse of a waveform 112 is applied from the source 32 through the conductor of a selected word line such as 18 to a source of reference potential such as ground. In response to the word pulse, the magnetic films 50 and 52 at the chain elements are rotated to or substantially to the position of the arrows 100, 104, 110 and 114. As shown by a waveform 116, a stored one in the chain element 16 provides a positive pulse 114 on the sense lead 36 and a stored zero provides a negative pulse 118 on the sense lead 36, the direction of rotation of a stored one or a zero providing the sensed polarity. Similarly, pulses are sensed in other digit leads such as 38 and 40 of the selected word line. At a time t the current pulse of the waveform 112 is terminated and the magnetic elements return to an orientation along their easy axis with the previously stored and interrogated one or zero polarity.

The areas 91 and 93 of FIG. 3 are substantially undisturbed by the read current so to aid the film to return to its stored state. Also the read current is selected to be of a magnitude so that the films do not rotate a full degrees. The magnetic state in the legs is rotated in response to the read current but returnsto the direction of arrows 93 and at time t The zero or one states may be sensed after passing through transformertype sense amplifiers (not shown) as is well known in the art. Between times t and t a period may be provided for recovery of circuit elements.

At a time t which is the start of the write period, current pulse of a waveform 120, may be applied to the conductor 54 of the selected word 18 to rotate the magnetic regions in the films 50 and 52 to the directions of arrows 100, 110, 112 and 114 substantially to saturation. At the same time a digit current pulse 122 or 124 of a waveform 126 is applied to each digit line of the selected word, such as line 36 to selectively write a one or a Zero. Thus the rotated magnetic elements are tilted in a selected direction of the arrow 80 or 88 (for film 50) and the arrows 82 and 90 (for film 52) to write a one or a zero in each chain element. At time t the word current pulse of the waveform 120 is terminated and the magnetic states are then established in the chain elements. Shortly after the time t the digit current pulse of the waveform 126 is terminated and at time t, which is after a short recovery period, a positive read current of the waveform 112 may be applied to a selected word. It is to be noted that the polarity of the read and write word current pulses are constant and only the digit current changes in direction.

In the arrangements of FIGS. 1 and 3 the digit-sense leads may be wound to be continuous through several columns of chain elements. It is to be noted that the digit current during writing does not permanently affect the stored state of unselected word lines. Because the chain elements are symmetrical, a selected state such as a one stored in any line as a result of a pulse of a selected polarity on a digit line 18 or 26 provides a sense signal of the same polarity at the source 34 when the corresponding word in interrogated.

Thus there has been described a chain magnetic memory system in which parallel thin films are selected to provide a desired demagnetization force with rotationaltype switching. The system operates with a minimum of driving current while developing a relatively large sense signal. The structure allows rapid formation and assembly of elements so that large capacity memories can be reasonably provided. A nondestructive reading results from the chain memory in accordance with the invention.

What is claimed is:

1. A chain magnetic memory element for rotational switching comprising:

a first conductor being substantially fiat and having first and second sides with a selected opening therebetween to form first and second structures each having first and second edges, said first conductor having a selected thickness,

first and second thin films positioned on the respective first and second sides with a gap therebetween at said first and second edges of said first and second structures, said first and second films having a shape anisotropy in a direction around said opening, the material of said films and said opening selected so that said shape anisotropy is equal to or greater than a magnetic field which would provide domain wall movement, said thickness selected to provide a substantially minimum demagnetization force while retaining rotational switching.

means for providing a first current through said first conductor to apply a rotational field to said first and second thin films,

a second conductor passing through said openings,

and means to apply a second current through said second conductor in a selected direction in time coincidence with said first current to establish a selected magnetic state in said thin films.

2. A chain storage element operable with rotational switching comprising:

an elongated conductive strip having first and second flat sides and having a plurality of widened portions at spaced intervals therealong with an opening in each widened portion between said first and second sides to provide first and second conductive portions on each side of the opening, said strip having a predetermined thickness,

first and second thin films respectively deposited on said first and second sides of said conductive strip and each having a magnetic shape anisotropy around each one of said openings sufiicient for rotational switching, said first and second thin films separated by said predetermined thickness selected to provide substantially a minimum demagnetizing force between said films while being operable with thin film rotational switching,

conductive means passing through each of said openmgs,

first means for passing current through said conductive strip for reading and writing to provide rotational fields in the films around each opening,

and second means for passing currents through said conductive means for writing to establish selected magnetic states around said openings.

References Cited UNITED STATES PATENTS 3,371,327 2/1968 Anderson et al. 34-'O--l74 3,378,821 4/1968 Leilich 340-174 3,379,539 4/l968 McGrath et al. 340-474 X OTHER REFERENCES Publication I: IBM Journal, September-November 1965, pp. 412417.

Publication II: IBM Technical Disclosure Bulletin, vol. 8, No. 6, November 1965, p. 886.

Publication HI: IBM Technical Disclosure Bulletin, vol. 8, No. 12, May 1966, p. 1817.

BERNARD KONICK, Primary Examiner S. B. POKOTILOW, Assistant Examiner 

